1. Field of the Invention
The present invention relates to a current sensor that detects a current flowing through a current path, and in particular, relates to a current sensor that detects magnetism generated when a current flows through a current path having a U-like shape.
2. Description of the Related Art
There have been well known current sensors that are each subsequently attached to an existing current path so as to control or monitor various types of device. It has been well known that, as the current sensor of this type, a magnetic sensor utilizing a magnetoelectric conversion element such as a magnetic resistance element or a Hall element, which senses a magnetic field generated from a current flowing through a current path is used.
There has been generally known a current sensor of a type out of the above-mentioned current sensors, in which a conductor (current path) having a U-like shape and a magnetic sensor are combined and fitted between current paths intended to be measured. As the current sensor of this type, such a current sensor 901 as illustrated in FIGS. 18A and 18B and FIGS. 19A and 19B has been proposed in Japanese Unexamined Patent Application Publication No. 2008-298761. FIGS. 18A and 18B are diagrams explaining the current sensor 901 of the related art, FIG. 18A is the perspective view thereof, and FIG. 18B is the cross-sectional view thereof. FIGS. 19A and 19B are diagrams explaining the current sensor 901 of the related art, FIG. 19A is a plan view illustrating a current sensing device portion 906, and FIG. 19B is a schematic configuration diagram illustrating the bridge circuit of the current sensing device portion 906.
The current sensor 901 illustrated in FIGS. 18A and 18B includes a primary conductor 903 having a U-like shape, a case 904 with a structure of being integrated with the primary conductor 903, and the current sensing device portion 906 arranged in a sensor substrate 902 provided within the case 904. In addition, if a current to be measured is applied to the primary conductor 903, induction magnetic fields M1 whose rotations are left-handed and right-handed are generated symmetrically with respect to a center line CL as illustrated in FIG. 18B, the induction magnetic fields M1 are sensed by the current sensing device portion 906, and a current flowing through the primary conductor (current path) 903 is detected.
In addition, as illustrated in FIG. 19A, in the current sensing device portion 906, four magnetoresistance effect elements (magnetic resistance elements) 911 (911a, 911b, 911c, and 911d), a connecting current line 912 for establishing connection between the magnetoresistance effect elements 911, and four terminal portions 913 (913a, 913b, 913c, and 913d) for inputting and outputting are included and provided on an installation board 914, and the magnetoresistance effect element 911a and magnetoresistance effect element 911b and the magnetoresistance effect element 911c and magnetoresistance effect element 911d are equally arranged in a line symmetrical manner in two respective regions into which the installation board 914 is divided by the center line CL.
In addition, the four magnetoresistance effect elements 911 are mutually arranged in directions parallel to the center line CL of the installation board 914, the magnetoresistance effect element 911a and the magnetoresistance effect element 911d are arranged in such orientations (DA and DD illustrated in FIG. 19A) of having magnetoresistance effect characteristics in which the resistance values thereof increase with increases in the induction magnetic fields M1, and the magnetoresistance effect element 911b and the magnetoresistance effect element 911c are arranged in such orientations (DB and DC illustrated in FIG. 19A) of having magnetoresistance effect characteristics in which the resistance values thereof decrease with increases in the induction magnetic fields M1. In addition, as illustrated in FIG. 19B, by establishing connection between the four magnetoresistance effect elements 911 (911a, 911b, 911c, and 911d) using the connecting current line 912, a bridge circuit 915 including a parallel connection of a first half-bridge circuit 916a and a second half-bridge circuit 916b is configured, the first half-bridge circuit 916a including a series connection of the magnetoresistance effect element 911a and the magnetoresistance effect element 911b, the second half-bridge circuit 916b including a series connection of the magnetoresistance effect element 911c and the magnetoresistance effect element 911d. In addition, while not illustrated, the terminal portion 913a is connected to an input terminal, the terminal portion 913b is connected to a ground terminal, and the terminal portion 913c and the terminal portion 913d are connected to signal (output) terminals.
In the current sensor 901 configured in such a way as described above, when such induction magnetic fields M1 as illustrated in FIG. 18B are generated, the directions of the induction magnetic fields M1 are opposite to each other in the first half-bridge circuit 916a and the second half-bridge circuit 916b. Therefore, the resistance values of the magnetoresistance effect element 911a and the magnetoresistance effect element 911d increase, and the resistance values of the magnetoresistance effect element 911b and the magnetoresistance effect element 911c decrease. From this, changes (increases or decreases) in respective electric potentials of an output voltage V1 from the terminal portion 913c and an output voltage V2 from the terminal portion 913d are opposite to each other. Therefore, it is possible to obtain a larger output signal by performing differential processing. In addition, in a case where external magnetic fields of the same amplitude are incident upon the four magnetoresistance effect elements 911 in the same direction, changes in the electric potentials of the output voltage V1 and the output voltage V2 are equal to each other. Therefore, it is assumed that it is possible to cancel the influences of the external magnetic fields by performing the differential processing.
By the way, as a magnetoelectric conversion element such as the above-mentioned magnetoresistance effect element 911, there is a magnetoelectric conversion element that has a sensitivity-influencing axis direction in which an output signal is influenced by reception of a magnetic field in a direction other than the sensitivity axis directions (DA, DB, DC, and DD illustrated in FIG. 19A). FIG. 20 is a diagram explaining a comparative example, and is a schematic configuration diagram compared based on the bridge circuit of the current sensing device portion illustrated in FIG. 19B. In addition, all magnetic resistance elements 93 (93A, 93B, 93C, and 93D) in the drawing illustrate a case where sensitivity-influencing axis directions (DAs, DBs, DCs, and DDs illustrated in FIG. 20) lie in directions rotated 90 degrees to the right with respect to the sensitivity axis directions (DA, DB, DC, and DD illustrated in FIG. 20).
In such a case, as illustrated in FIG. 20, if an external magnetic field MY in a Y-axis direction is incident, it is difficult to cancel the influence of the external magnetic field MY by even performing differential processing on the output voltage V1 and the output voltage V2. In other words, even if the constant induction magnetic fields M1 are generated by the current to be measured flowing through the conductor 92, the induction magnetic fields are sensed as if being further changed, owing to the external magnetic field MY in the Y-axis direction, and the value of a current flowing through the conductor 92 is detected as a different value. In this way, in a case where the magnetic resistance elements 93 having the sensitivity-influencing axis directions are used, it is difficult for the configuration of the related art to cancel the influence of the external magnetic field MY, and there has been a problem that it is difficult to obtain a current sensor having good accuracy.
The present invention solves the above-mentioned problem, and provides a current sensor in which the influence of an external magnetic field is reduced to obtain good detection accuracy.